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<a name="Thread_002dLocal-Storage"></a>
<h3 class="section">6.64 Thread-Local Storage</h3>
<a name="index-Thread_002dLocal-Storage"></a>
<a name="index-TLS"></a>
<a name="index-_005f_005fthread"></a>

<p>Thread-local storage (<acronym>TLS</acronym>) is a mechanism by which variables
are allocated such that there is one instance of the variable per extant
thread.  The runtime model GCC uses to implement this originates
in the IA-64 processor-specific ABI, but has since been migrated
to other processors as well.  It requires significant support from
the linker (<code>ld</code>), dynamic linker (<code>ld.so</code>), and
system libraries (<samp>libc.so</samp> and <samp>libpthread.so</samp>), so it
is not available everywhere.
</p>
<p>At the user level, the extension is visible with a new storage
class keyword: <code>__thread</code>.  For example:
</p>
<div class="smallexample">
<pre class="smallexample">__thread int i;
extern __thread struct state s;
static __thread char *p;
</pre></div>

<p>The <code>__thread</code> specifier may be used alone, with the <code>extern</code>
or <code>static</code> specifiers, but with no other storage class specifier.
When used with <code>extern</code> or <code>static</code>, <code>__thread</code> must appear
immediately after the other storage class specifier.
</p>
<p>The <code>__thread</code> specifier may be applied to any global, file-scoped
static, function-scoped static, or static data member of a class.  It may
not be applied to block-scoped automatic or non-static data member.
</p>
<p>When the address-of operator is applied to a thread-local variable, it is
evaluated at run time and returns the address of the current thread&rsquo;s
instance of that variable.  An address so obtained may be used by any
thread.  When a thread terminates, any pointers to thread-local variables
in that thread become invalid.
</p>
<p>No static initialization may refer to the address of a thread-local variable.
</p>
<p>In C++, if an initializer is present for a thread-local variable, it must
be a <var>constant-expression</var>, as defined in 5.19.2 of the ANSI/ISO C++
standard.
</p>
<p>See <a href="https://www.akkadia.org/drepper/tls.pdf">ELF Handling For Thread-Local Storage</a> for a detailed explanation of
the four thread-local storage addressing models, and how the runtime
is expected to function.
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